1. Field of the Invention
The invention relates in general to the fabrication of dynamic random access memory (DRAM) capacitor, and more particularly to applying tungsten nitride (WN) in DRAM capacitors.
2. Description of the Related Art
The conventional structure of capacitors is stacked with metal-insulator-silicon (MIS). A polysilicon layer used as the electrode of a capacitor oxidizes easily at its surface to form a native oxide layer. The native oxide layer decreases the dielectric constant of a dielectric layer and reduces its capacitance, thereby decreasing the quality of the dielectric layer.
In response, a conventional method improves the problem described above by using a metal layer to replace the polysilicon layer. This means a capacitor structure stacked with metal-insulator-metal (MIM) is provided. Furthermore, the capacitor structure is used in fabrication of a nonvolatile ferroeletric memory (FeRAM) and a capacitor with a high dielectric constant, as used in highly integrated DRAM.
Nevertheless, it is difficult to directly fill a metal material into a contact in the conventional structure of the MIM capacitor. Since the step-coverage of metal is imperfect, a polysilicon layer and a barrier layer (or an adhesion layer) must be provided to fill the contact before providing the metal layer as a bottom electrode of the capacitor. This makes the process more complex. Also, the surface area of the bottom electrode is increased for improved capacitance. The metal is usually thick to achieve the object described above. But forming the thick metal layer induces a micro-loading effect that makes the etching process more complex and time-consuming.
Tungsten nitride is suitable for use in the metal layer of the capacitor because of its great oxidation resistance and for compatibility with chemical vapor deposition process. However, when tungsten nitride comes directly into contact with polysilicon, tungsten nitride may peel due to poor adhesion between tungsten nitride and polysilicon. Therefore, it is necessary to provide a diffusion barrier layer between tungsten nitride and polysilicon.
Additionally, a conventional method of forming tungsten nitride provides performance of a rapid thermal process after implanting nitrogen into tungsten silicide. But tungsten nitride formed in this method has a slow growth rate and has poor uniformity, which decreases the conductivity of the tungsten nitride.
It is therefore the object of the invention to provide an improved and simplified method of fabricating DRAM capacitors. This method uses tungsten nitride as the MIM capacitor structure.
The invention achieves the above-identified objects by providing a method of fabricating DRAM capacitors. Tungsten nitride is used in fabrication of capacitors having a simple structure. Also, the process of forming the capacitor is easily performed. A doped polysilicon layer is provided, in a portion smaller than a bottom electrode of a capacitor. Then, a tungsten silicide layer is formed, and a tungsten nitride layer is formed on the surface of the tungsten silicide layer.